Method of collecting gases in aluminum furnaces



Oct. 24, 1950 R. JOUANNET 2,526,875

I METHOD OF COLLECTING GASES IN ALUMI-NUM FURNACES Filed June 19, 1948 2Sheets-Sheet 1" k I INVENTOR Fofiar/ ffluanm'f ATTORNEY R. JOUANNET2,526,875

METHOD OF COLLECTING GASES IN ALUMINUM FURNACES Oct. 24, 1950 2Sheets-Sheet 2 Filed June 19, 1948 V INVENTOR. 170%? jbuamwf BY PatentedOct. 24, 1950 METHOD OF COLLECTING GASES IN ALUMINUM FURNACES Robert Jouannet, La Praz, near Modane,jFrance, assignorto Elektrokemisk A/S, a.corporation of Norway Application June 19, 1948, Serial No. 33,955 InFrance January 21, 1944 Section 1, Public Law 690, August 8, 1946 Patentexpires January 21, 1964 4 Claims. (01. 204-67) In the reduction ofalumina to produce aluminum, a low voltage current is passed through abath of cryolite in which the alumina is dissolved. A carbon or graphiteelectrode is employed and the electric current both supplies the powerfor electrolysis and generates heat to maintain the bath in a moltencondition. The oxygen driven off from the alumina goes to the carbonelectrode where it combines to form a mixture of CO andCOz. In addition,there is always some decomposition of the oryolite so that the gasesgiven off contain fluorine compounds.

The method of handling this gas has for many years been recognized asone of the problems in the production of aluminum. If an open furnace isemployed, the pot room becomes fouled by the gases given off, andworkingconditions are bad. Enclosing the furnace only gives partial benefit, asit is necessary for the workmen to get at the surface of the bath bothfor the introduction of additional quantities of alumnia and in order tobreak up the crust which forms on the surface of the bath. In such caseit is impossible to make the enclosure for the furnace air-tight and thegases that are drawn off are highly diluted with air, so that handlingthem is cumbersome and expensive. The difiiculty of this problem isincreased by the fact that great care must be taken to prevent any ironentering the bath (iron is a highly undesirable impurity in aluminum),and the temperature adjacent the surface of the bath is so high that itwould bar the use of aluminum shields.

I have discovered that this problem can be solved as follows: One of thestandard methods of producing aluminum involves the use of continuouselectrodes which are fed slowly into the bath as they are consumed. Suchelectrodes, being fluid at the top, are provided with a sheath whicheither moves with the electrode (in such case it is ordinarily made ofaluminum) or the sheath is fixed so that the electrode moves through it.In some instances the two types of sheath are both employed. Accordingto my invention, a gas-collecting ring member is supplied whichsurrounds the electrode; a gas-tight connection is made between itsupper portion and the sheath and the lowerportion is spaced asubstantial distance away from the electrode, This forms a pocketbetween such member and the electrode in which gas can be collected. Ihave also discovered that a substantially gas-tight connection can bemade between this ring member and the bath without short-circuiting thepot and without causing undue contamination by bringing the lower edgeof this member down close to the crust of the bath but slightly aboveit. The crust on the bath as well as the bath itself is an electricalconductor and it is important that this member should not make contactwith the crust. V

The final seal between the crust and the lower edge of the apron of thisring member is made by packing powdered alumina against the edge of suchapron and piling it on top of the crust of the bath. Powdered alumina isa very poor conductor of heat and is substantially a nonconductor ofelectricity.

The connection between the ring member and the sheath may be made invarious manners. For example, if the sheath is a fixed one, the ringmember may be attached to it directly as by welding. Ordinarily,however, it is better to make a joint between the ring member and thesheath which permits relative movement between them. This permits theuse, of a movable sheath and even when a fiXed sheath is employed, itpermits the ring member to be lifted so that access can be had to thelowerpart of the electrode.

I have found that the best way to make a connection between the ringmember and the sheath which permits relative movement is to bring thering member either against or adjacent the sheath and arrange a pocketso that alumina can be packed to .fill the joint between thetwo. If thespace between the sheath and the ring memberis relatively large, smallchunks of alumina can first be inserted andthen fine material can be puton top; or if the space is small, the whole packing may be formed offinely-powdered alumnia. Of course, other sand-like material can beemployed but as there, isa possibility of this material getting into thebath, it is preferable to use alumina, which can do no harm.

While this invention is particularly useful in connection with pots forthe production of aluminum, it may also be employed with equipment forany other similar process where like conditions are found.

This invention can readily be understood by reference to theaccompanying drawings in which Fig. 1 is a transverse section throughsuch a furnace embodying my invention; Fig. 2 is a detailed view showingthe connection between the ring member and the fixed sheath and alsoshowing the manner in which the gases are withdrawn from the pocketbetween the ring member and the electrode; Fig. 3 is a section on line33 of Fig. 2 and Figs. 4, 5 and 6 are detailed views indicatingalternative forms of construction.

In the drawings, the pot I0 is of usual construction. The electrode [2is of the continuous self-baking type, sometimes termed the Soederw bergtype and goes down into the pot as indicated in Fig. 2. The electrodemoves in a fixed casing !4 reinforced by channels l5. The electrode issuspended by contact rods 18 which go down into the electrode mass. Thedetailed manner in which these rods are handled is explained more fullyin my co-pending application Serial No. 25,174, filed May 5, 1948, nowUnited States Patent No. 2,475,452, issued July 5, 1949. The contactmembers [8 are attached to support bars 20 and these in turn are clampedto bus bars 22 which also serve to carry the weight of the electrode.The bus bars 22 can be raised and lowered by jacks 24. The broken linethrough the mass of the electrode l2 indicates approximately the zonewhere the electrode material has become baked and solidified.

Coming now to my present invention, which is shown in detail in Figs. 2and 3, a member 3!] is bolted to the bottom channel [6 and entirelysurrounds the electrode so that in effect it is part of the fixed casingor sheath. It is to be noted I that this member is shaped to form acontinuous pocket or groove around the electrode. The ring member inthis instance comprises an apron of corrugated or fluted metal 32 (seeFig. 3) to which is welded a top plate 34. The inner edge of the plate34 has a downwardly depending flange 35 which can go down into thegroove or pocket in the member 30. At spaced intervals flanges 38 arewelded to the plate 34 for suspension of the ring member.

A shaft 40 runs down each side of the furnace and each such shaft iscarried by brackets 42 attached to the upper channel l6. These shaftsare rotatable and carry drums 44 for winding up suspension chains 46which are attached to the flanges 38 of the ring members. If desired,the strips 32 can be interrupted on each side of the electrode toprovide a sight box 48 which has a removable cover 48. Such boxes mayalso be used for introducing alumina within the line of the ring member.

In the present embodiment of my invention, an exhaust pipe 50 is weldedinto the ring member 32 at some convenient point. This pipe 50 is inturn welded into an inverted cup 52 which fits down into a circularrecess formed in the upper surface of the gas-receiving chamber 54. Thegas which passes down into the gas-receiving chamber 54 may then beburned as under the hood 56 to destroy tar vapors from the electrode,and the residual gas withdrawn through the pipe 85 to any convenientpoint where it may either be disposed of or treated to recover thefluorine products according to known processes. The groove in the member32 and the recess in the member 54 are packed with finely-powderedalumina to prevent gas leakage.

During the operation of the furnace, the aluminum goes to the bottom ofthe pot as indicated at 51 in Figs. 1 and 2. Above this is the moltenbath of cryolite containing dissolved alumina indicated at 58. The uppersurface of this bath (and to some extent its edge portions) tends tosolidify to form a crust indicated at 50. The upper layer of this crustis somewhat irregular and tends to merge into the loose alumina 6! aboveit but there is a distinct difference between the two as the fused crustis an electrical conductor, whereas the loose alumina is not. This crustmay rest on the surface of the bath or may have sufiicient strength tostay above this surface when molten aluminum is drawn off, lowering thesurface level of the bath. Apron 32 is so proportioned that its bottomedge comes down adjacent to the crust on the bath but does not enter itsufiiciently to make electrical contact. The loose alumina BI is thenpacked in between the edge of the apron 32 and the side of the pot 10 toform a substantially gas-tight seal.

In this case the seal is between the edge of the ring member and theside of the pot. In other examples it is between the edge of the ringmember and the crust. This distinction is relah tively unimportant. Theessential point is that there must be a substantial gas-impervious coverfor the bath (which cover may include the crust) and the apron must notmake electrical contact with the bath either through the crust or thepot as this may tend to short-circuit the furnace. Where the seal isbetween the ring member and the pot, the ring member must be raised (atrather infrequent intervals) to introduce alumina unless special meansfor supplying the alumina is provided as described above.

In Fig. 4 a simple form of ring member 62 is indicated suspended by achain or cable 54. In this case instead of packing the joint between thesheath 5t and the ring member 62 with alumina, a V-shaped metal piece 68is dropped into the pocket between the members 62 and 66. This view alsoillustrates the manner in which solidlfied materials (perhaps solidifieddrops from the bath carried up with the escaping gases) tend to build upon the inside of the ring member if its sides are vertical as shown inthis figure. Because of that it is preferable to have the ring memberflared away from the electrode as shown in Figs. 1 and 2.

Fig. 5 shows a structure having a movable sheath l0 and a ring member 12suspended by chain 14. The permanent sheath is indicated at T6. In thiscase the joint between the member 72 and the movable sheath 10 is packedwith powdered material as indicated at 18. As an appreciable clearancebetween members 12 and 1D is here indicated, this packing may be made upof pieces of appreciable size made by breaking up a part of the crust orusing the alumina where it is partly sintered adjacent the crust. Thismay be supplemented with finely-powdered alumina.

Fig. 6 shows a ring member welded directly to the permanent sheath 82 sothat no such suspension member is needed. The pipe 34 is used forwithdrawing gases. Since this ring member cannot be raised, ordinarilythe clearance between its lower edge and the crust should be greaterthan is otherwise necessary so that by inserting a bar at an angle, thecrust underneath this ring member may be broken and alumina in--troduced. It is understood that with any of the forms of device shown itis customary (as in the usual operation of an aluminum pot) to break thecrust periodically. This causes the loose alumina on the crust (withadditional alumina if desired) to enter the bath to be dissolved in themolten cryolite. In carrying out my invention conditions are then oncemore maintained to cause the crust to reform and again loose alumina ispacked against thebottom'of the gas-collecting ring to; form thedesiredsubstantially gas-tight joint between the ringandthe'crust.

I have previously stated. that the apparatus of this invention permitsme to withdraw the gases in a muchmore concentrated. state thanwas'previously possible. For. example, I am; familiar with theoperations carried out under United States Patent No. 2,031,554 havingto do with the recovery of gases from an aluminum reduction plant. Theoperation of that process was dependent upon using an enclosed furnace,but nevertheless the average gases removed contained less than 1% ofgases which came from the electrochemical process and there was a largedilution with air. According to the present invention during normaloperation there is virtually no air dilution.

One direct result of this ability to withdraw the gases in undilutedform is that they are sufliciently concentrated so that the combustileelements in them may be burned. This is important since the hydrocarbontar vapors coming out from the lower part of the electrode mass(resulting from the decomposition of the binder used in the electrode)were extremely difficult to remove from the gases previously obtainedand would go through the scrubbing apparatus and have an injuriousaction on surrounding vegetation.

Experience has shown that the gases actually developed by the chemicalreaction contain a major percentage of carbon dioxide. For example, theratio of CO to CO2 may equal approximately 40/60. In this ratio theremight be a tendency for the gases moving up around the electrode tocorrode the sides of the electrode under the thermal conditionsobtaining, following the reaction CO2+C=2CO. It may be noted that thetemperature of the bath is about 950 C. and while the tar vapors will bemuch cooler, the temperature in the pocket between the ring member andthe electrode will range between 950 and about 500 C. It has been foundthat when one entrains the tar vapors directly with the gases evolvedfrom the reaction, some of the carbon from the tar appears to react withthe gases generated below the surface of the bath, so that the resultingproportion of CO to CO2 may be increased to about 60/40. This tends toprotect the side of the electrode against corrosion and of course at thesame time the side of the electrode is protected against direct exposureand combustion in the air. Also this makes a gas mixture which canreadily be burned for the destruction of tars, as previously stated.

Summing the matter up, the use of this apparatus permits me to carry outa process (when the furnace is in full operation and air has beendisplaced from the gas passages) in which the gases resulting from theprimary process of reducing the aluminum are blended without admixtureof substantial quantities of oxygen with the hydrocarbons resulting fromthe decomposition of tars in the electrode, and this blanding takesplace while the gases are still hot enough to be conducive to theformation of CO in the presence of excess hydrocarbons from the tars. Thresulting gases are then burned with admixture of air to decompose anytar vapors remaining and finally the gases are subjected to a knownprocess for scrubbing out fluorine compounds.

In the foregoing specification, I have indicated the best method knownto me of carrying out my invention and have indicatedsome modificationsthat may be employed. However, many other modifications may be madewithout departing.

from the spiritoimy invention.

This application is a continuation in part of my earlier application,Serial No. 587,514, filed April 10, 1945, for. Open. Electrolytic Cellwith Device for CollectingGasesj. now abandoned.

What I claim is:

1. The method of operating an aluminum furnace of a type comprising apot for the furnace charge, a casing for guiding an electrode downtowards the pot, a carbonaceous electrode in said casing, agas-collecting ring member spaced out from the casing at the lower edgeof the ring member and means for making a substantially gas-tight jointbetween the casing and an upper part of the ring, which method comprisesthe steps of maintaining within the pot a charge of alumina dissolved inmolten cryolite under conditions to cause a crust to form on such'chargereaching up to a level approaching but not contacting saidgas-collecting ring so that no substantial electrical discharge is madethrough such ring, piling substantially electrically non-conductivealumina on such crust in quantity sumcient to embed the lower edge ofsuch ring thus forming a gas-tight joint between the charge and thegas-collecting ring, collecting gases within such ring, and withdrawingsuch gases from within the ring.

2. A method as specified in claim 1 which includes the further step ofburning the gases collected within the ring, with air.

3. A method as specified in claim 1 in which said electrode is of thecontinuous self-baking type and in which the gases collected within thering comprise tar vapors from the electrode and CO and CO2 evolvedWithin the bath and in which conditions are maintained so that thetemperature of a mixture of such gases is within the range of between500 C. and 900 C. so that a reaction takes place between tar vapors andCO2 to increase the proportion of CO in the mixture.

4. The method of operating an aluminum furnace of a type comprising apot for the furnace charge, a casing for forming and guiding anelectrode down towards the pot, a continuous selfbaking type ofcarbonaceous electrode in said casing extending down into the pot belowthe lower edge of the casing, a gas-collecting ring member having itslower edge spaced out from the electrode to form a gas-collecting areaaround the electrode and having an exhaust pipe for withdrawing gasesfrom within such area, and means for making a substantially gas-tightjoint between such ring member and the casing, which method comprisesthe steps of maintaining within the pot a charge of alumina dissolved inmolten cryolite, maintaining such charge under conditions to cause acrust to form thereon which reaches up to a level approaching but notcontacting said gas-collecting ring, piling alumina on said crust inquantity sufiicient to embed the lower edge of such ring so that agas-collecting chamber is formed around the lower portion of theelectrode above the charge which chamber is substantially gas-tightexcept for said exhaust pipe, collecting within such chamber gases fromthe bath and vapors from the electrode substantially without admixturewith air, withdrawing such gases and vapors from the chamber and burningthe same.

ROBERT JOUANNET.

(References on following page) REFERENCES CITED FOREIGN PATENTS Thefollowing references are of record in the Number Country Date file ofthis patent: 361,683 France Aug; 9, 1906 UNITED STATES PATENTS OTHERREFERENCES Number Name Date Journal of the Electrochemical Society, vol.94,

2,031,554 Torchet Feb. 18, 1 3 No. 5, November 1948, pages 220 to 231,

2,100,927 Trematore Nov. 30, 1937

